1. Field of the Invention
The present invention relates to a novel aromatic polycarbodiimide and a water repellent sheet made therefrom. More particularly, the present invention relates to an aromatic polycarbodiimide having high heat resistance, low hygroscopicity and a low dielectric constant and to a water repellent sheet made from such an aromatic polycarbodiimide.
2. Description of the Related Art
There have been known aromatic polycarbodiimides produced by polymerizing diphenylmethane diisocyanate (MDI) or tolylene diisocyanate (TDI) as a monomer. These aromatic polycarbodiimides are used as flame resistant films or heat resistant adhesives because of their excellent heat resistance.
From the point of view that these aromatic polycarbodiimide films do not generate a volatile gas or decomposed monomer even when they are exposed to a temperature higher than 400.degree. C., it can be said that these aromatic polycarbodiimide films have acceptable heat resistance. However, they have low moisture resistance, and loses self supporting properties and become fragile when they are heat-treated at 200.degree. C. or more, so that they are not fit for practical use. In addition, they have poor solubility in an organic solvent and low workability.
Further, recently it is required for heat resistant films to have various properties in combination depending on their use. For example, in the case of interlayer insulating films for printed circuit boards or LSIs, low thermal expansion coefficient and low dielectric constant are expected. For components for use in optical communication, in particular cladding materials for use in optical waveguides, low refractive index is expected. Furthermore, to keep the values of physical properties constant, water absorption must be low. When heat resistant films are used as a surface protector for molded articles used under humid atmospheres or as a heat resistant releasing material, it is effective to impart water repellency thereto.
For example, polyimides, which are excellent in heat resistance and they are used widely, have a high water absorption due to the fact that they contain in their molecule many imide groups, which are highly polarized. On the other hand, in the case where they contain fluorine in the molecule, polyimides have a small contact angle, so that no sufficient water repellency can be obtained.
Water repellency can be imparted by various methods, which include methods in which fluorochemical compositions are added, methods in which fluorine atom is introduced in the polymer main chain, etc. As one of the methods in which fluorochemical compositions are added, there has been a report on the use of a carbodiimide compound as an extender (Japanese Patent Application Laid-open No. Hei 8-325220). Here, the carbodiimide compound is used as an extender for fluorochemical compounds in order to reduce costs but it does not exhibit water repellency or oil repellency when used alone.
As the polymer having introduced in the polymer main chain, there has been known polytetrafluoroethylene (PTFE), which however has a somewhat poor processability. Also, there has been known a method in which a substituent containing many trifluoromethyl groups is introduced to one or both ends of polyimide molecule. However, this method is disadvantageous from the viewpoint of costs (Japanese Patent Application Laid-open No. Hei 11-21350).